An internal combustion engine has many sources of energy loss, most of which are considered to be sacrificed in order to obtain a controlled combustion and to provide an output that can be converted to perform useful work. Such losses, such as heat losses and frictional losses can be minimized by the design of the engine. Usually, an engine design reflects a balance of different energy loss considerations that are reached to meet overall design criteria. The compromises that are made and the factors that are considered in designing an internal combustion engine for a particular application are well known to engineers.
Although losses in energy and efficiency are minimized by the design of an internal combustion engine, or compromised in view of the type of work that the internal combustion engine is intended to perform, other energy and efficiency losses are present that are generally not considered to be recoverable. For example, during combustion a piston is typically driven to rotate a crankshaft by applying the force of the combustion to the head of the piston and transmitting this force in the direction toward the crankshaft through the piston connecting rod during the power or work stroke of the piston. According to Newton's third law, however, ("To every action there is an equal and opposite reaction")there is a force (reactive force) equal and opposite to the force that drives the piston in the direction toward the crankshaft, and this force is absorbed by the block and head of the typical internal combustion engine. Since the engine is usually mounted in a fixed arrangement, the reactive force is not utilized to perform useful work.